Morphological Diversity and Relationships in the A-Genome Cottons, Gossypium arboreum and G. herbaceum

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1 Botany Publication and Papers Botany Morphological Diversity and Relationships in the A-Genome Cottons, Gossypium arboreum and G. herbaceum Marsha A. Stanton University of Arkansas - Main Campus J. McD. Stewart University of Arkansas - Main Campus A. Edward Percival United States Department of Agriculture Jonathan F. Wendel Iowa State University, jfw@iastate.edu Follow this and additional works at: Part of the Botany Commons, Genetics Commons, and the Plant Breeding and Genetics Commons Recommended Citation Stanton, Marsha A.; Stewart, J. McD.; Percival, A. Edward; and Wendel, Jonathan F., "Morphological Diversity and Relationships in the A-Genome Cottons, Gossypium arboreum and G. herbaceum" (1994). Botany Publication and Papers This Article is brought to you for free and open access by the Botany at Iowa State University Digital Repository. It has been accepted for inclusion in Botany Publication and Papers by an authorized administrator of Iowa State University Digital Repository. For more information, please contact digirep@iastate.edu.

2 Morphological Diversity and Relationships in the A-Genome Cottons, Gossypium arboreum and G. herbaceum Abstract The Asiatic or A-genome cottons, Gossypium arboreum L. and G. herbaceum L., are potentially important genetic resources for cotton breeding programs. The National Plant Germplasm System (NPGS) contains approximately 4 accessions of these species, but little information is available on the diversity within the collection or on characteristics of individual accessions. This investigation was initiated to provide a morphological description of each accession. These data were used to evaluate the range of diversity represented within the collection and to address the questions of species and race distinctions. Multivariate techniques were used to assess similarities among accessions and to evaluate morphological parameters contributing to the variation in each species. Means for 41 of 53 characters were significantly different between species, although high intraspecific variability resulted in range overlap for all characters. Principal component analysis separated the two species. Accessions from southern Africa and racial designations of africanum and wightianum formed clusters within G. herbaceum based on the first two principal components; no clusters were noted within G. arboreum. Accordingly, the validity of intraspecific or racial classifications for most of the accessions of this latter species in the current NPGS collection is questionable. Additional germplasm acquisitions from under- or non-represented areas could expand genetic diversity in the collection. Keywords Gossypium herbaceum, G. arboreum, tetraploid cotton species, National Plant Germplasm System, Germplasm Resources Information Network, principal component analysis Disciplines Botany Genetics Plant Breeding and Genetics Comments This article is from Crop Science 34 (1994): 519, doi:1.2135/cropsci x34239x. Rights Works produced by employees of the U.S. Government as part of their official duties are not copyrighted within the U.S. The content of this document is not copyrighted. This article is available at Iowa State University Digital Repository:

3 Published March, 1994 PLANT GENETIC RESOURCES Morphological Diversity and Relationships in the A-Genome Cottons, Gossypium arboreum and G. herbaceum Marsha A. Stanton, J. McD. Stewart,* A. Edward Percival, and Jonathan F. Wendel ABSTRACT The Asiatic or A-genome cottons, Gossypium arboreum L. and G. herbaceum L., are potentially important genetic resources for cotton breeding programs. The National Plant Germplasm System (NPGS) contains approximately 4 accessions of these species, but little information is available on the diversity within the collection or on characteristics of individual accessions. This investigation was initiated to provide a morphological description of each accession. These data were used to evaluate the range of diversity represented within the collection and to address the questions of species and race distinctions. Multivariate techniques were used to assess similarities among accessions and to evaluate morphological parameters contributing to the variation in each species. Means for 41 of 53 characters were significantly different between species, although high intraspecific variability resulted in range overlap for all characters. Principal component analysis separated the two species. Accessions from southern Africa and racial designations of africanum and wightianum formed clusters within G. herbaceum based on the first two principal components; no clusters were noted within G. arboreum. Accordingly, the validity of intraspecific or racial classifications for most of the accessions of this latter species in the current NPGS collection is questionable. Additional germplasm acquisitions from under- or non-represented areas could expand genetic diversity in the collection. M.A. Stanton and J.McD. Stewart, Dep. of Agronomy, 115 Plant Science Bldg., Univ. of Arkansas, Fayetteville, AR 7271; A.E. Percival, USDA, ARS, Route 5, Box 85, College Station, TX 77845; and J.F. Wendel, Dep. of Botany, Iowa State Univ., Ames, la 511. Published with the approval of the Director of the Arkansas Agric. Exp. Stn. Received 14 Dec *Corresponding author. Published in Crop Sci. 34: (1994). THE GENUS Gossypium L. contains four cultivated species: the New World allotetraploids (2n = 4x = 52), G. barbadense L. and G. hirsutum L., and the Asian-African diploids (2n = 2x = 26), G. arboreum L. and G. herbaceum L. These latter two species are collectively called a variety of terms including Old World, Asiatic, and A-genome cottons. The last designation will be used herein because it most accurately defines the two species. Allotetraploids (AD genome) currently are responsible for 95% of world cotton production, having nearly displaced A-genome species from the range in which they were previously cultivated (Meredith, 1991). Gossypium arboreum (Az) is still cultivated on marginal land in Pakistan and India, with active breeding programs emphasizing its use in non-woven materials (M. Akhtar, 1992, personal communication). Limited cultivation of G. herbaceum (At) occurs in Africa (Lee, 1984). Today, the primary value of the A-genome cottons is as a source of genetic diversity for the tetraploid cottons. With the development of techniques for introgression of A-genome germplasm into AD-genome cultivars (Stewart, 1992; Stewart and Hsu, 1978), these cottons have become a more readily usable germplasm resource for tetraploid cotton breeding programs. In past breeding efforts, A-genome cottons have con- Abbreviations: A~o Gossypium herbaceum; A 2, G. arboreum, AD, tetraploid cotton species; NPGS, National Plant Gerrnplasm System; GRIN, Gerrnplasm Resources Information Network; PC, principal component; PCA, principal component analysis.

4 52 CROP SCIENCE, VOL. 34, MARCH-APRIL 1994 tributed high fiber strength and resistance to Pectinophera gossypiella Saunders, Puccinia cacabata Arth. and Holw., and some races of Xanthomonas campestris pv. malvacearum to AD-genome cultivars (Fryxell, 1984; Meredith, 1991). Additionally, resistance to nematodes (Carter, 1981; Yik and Birchfield, 1984), fungal and bacterial pathogens (Mathre and Otta, 1967; Bollenbacher and Fulton, 1971), and insects (Benedict et al., 1987; Stanton et al., 1992) has been found in A-genome cottons. These sources of genetic resistance take on added importance since modem upland cultivars have a relatively narrow genetic base (Wendel et al., 1992) in spite of documented interspecific introgression (Endrizzi et al., 1985; Meredith, 1991). Morphological resemblance between G. arboreum and G. herbaceum makes individual specimens difficult to identify, especially if fruiting material is not available. Wendel et al. (1989) indicated that the mean values of 25 morphological characters differed significantly between the two species; but, infraspecific variability was sufficiently high that ranges of all morphological characters overlapped between species. At the chromosomal level, the species differ by a reciprocal translocation (Gerstel, 1953), and in chromosome length, symmetry, and number of secondary constrictions and satellites (Gennur et al., 1988). Measures of allozyme variability indicate that G. arboreum contains greater variability than G. herbaceum, and both species exhibit diversity roughly equivalent to that found among upland G. hirsutum cultivars (Wendel etal., 1989; 1992). This is only about half of the diversity normally found among cultivars of other domesticated plant species (Doebley, 1989). In addition to the decline in genetic diversity normally associated with domestication (Doebley, 1989), the dramatic decline in cultivation of A-genome cottons likely has resulted in additional erosion of genetic diversity. Variability within G. arboreum and G. herbaceum has been partitioned into infraspecific races based on a combination of geographical and morphological factors (Hutchinson and Ghose, 1937; Silow, 1944; Hutchinson, 195). These racial concepts were not intended to provide distinct morphological delimitations among the races, and assignment to a particular race is difficult for accessions of unknown origin. One of the described races, africanum, has been assigned to various rankings below the species level and is currently given sub-species rank (Vollesen, 1987). The Asiatic Cotton Germplasm Collection of the National Plant Germplasm System (NPGS) contains approximately 4 accessions, few of which have been characterized or evaluated (Percival, 1987). This study was initiated to provide morphological descriptions of the accessions and determine the range of diversity represented within the collection. The questions of species and race distinctions are also addressed. MATERIALS AND METHODS Plant Materials Seeds of most of the accessions in the NPGS Asiatic Cotton Germplasm Collection were obtained from the USDA, College Table 1. Definitions of morphological characters for Gossypium arboreum and G. herbaceum. Flower Petal color: 1 = white, 2 = cream, 3 = white wired edge, 4 = It. yellow, 5 = yellow, 6 = yellow wired edge, 7 = red. Petal spot: = none, 1 = white, 2 = cream, 3 = pink, 4 = red. Pollen color: 1 = yellow, 2 = cream. Petal length: distance from bract attachment to corolla edge at anthesis (em). Leaf Color: 1 = yellow-green, 2 = It. green, 3 = green, 4 = dk. green, 5 = green wired veins, 6 = red. Number of lobes: average number of lobes on 3 fully expanded leaves. Lm length: distance from petiole to tip of central lobe (em). Ls length: distance from petiole to tip of first lateral lobe (em). Lm width: width of central lobe at widest point (em). Lobe shape: Lm width!lm length(%). Lobe length: Ls length!lm lobe length(%). Si: distance from base of sinus to the mid point of an imaginary line connecting the tips of Lm and Ls (em). Sinus depth: Si/LM length(%). Glands: number of gossypol glands per 5 mm on main leaf vein counted between nectary and point of attachment of petiole. Stipule: average length of stipules on a fruiting branch (em). Bract Sunred: degree of light induced anthocyanin in bracts; = none to 5 =high. Length: distance from base of bract to tip of central tooth (em). Width: distance at widest point (em). Number of teeth: average number of teeth per bract. Teeth length: average length of three central teeth (mm). Nectaries Leaf and flower nectaries: presence = 1, absence =. Leaf nectary area: nectary length x width (mm 2 ). Pubescence Stem rating: = glabrous to 5 = very dense pubescence. Stem, 2 layers: 1 = present, = absent. Main stem: length of most prevalent layer of pubescence, measured above highest unrolled leaf. Minor stem: length of minor layer of pubescence, measured above highest unrolled leaf. Simple hairs: length of simple hairs on lower surface of mature leaf (mm). Stellate hairs: maximum length of stellate hair branches on lower surface of mature leaf (mm). Boll Length: perpendicular distance from apex to base (em). Width: diameter at point of maximum circumference (em). Seed and fiber Fuzz density: degree of fuzz fibers on seed; = none to 5 = very dense. Lint color: 1 = white, 2 = off-white, 3 = tan. Seed index: weight of 1 fuzzy seed (g). Naked seed weight: weight of 1 acid-delinted seed (g). Lint percent: ginned lint wt./wt. of seed cotton(%). E1 t: percent elongation at point of break in strength determination. Micronairet: fiber fineness (micronaire units). Tlt: fiber strength when clamped at 3.2 mm (kn m kg"'). 5% span Iengtht: length spanned by 5% of the fibers scanned at the initial starting point (mm). 2.5% span Iengtht: length spanned by 2.5% of the fibers scanned at the initial starting point (mm). At: external surface area of fibers measured at standard pressure (mm 2 1 mm'). AHt: external surface area of fibers measured at high pressure (mm 2 1 mm 3 ). Difference (D)t: AH - A, approximation of flatness of fiber (mm 2 1mm'). Maturityt: caustic soda percent maturity. Immaturity ratio (Ot: (.7* 1) 112 Perimetert: (12566*1A; (!JDl). Weight finenesst: (485*1'*1A 2 ; (pg). Wall thicknesst: 21[A(1 ( )); (!JDl). t From standard breeder's test, by Starlab, Knoxville, TN; for more complete descriptions of measurements see Regional Cotton Variety Tests (1991). Station, TX. Additional materials were obtained from working collections at the University of Arkansas, Fayetteville, AR, and Iowa State University, Ames, la. All accessions were field grown at Fayetteville, AR, in one or more of the years 1987 through Many of the accessions were also grown

5 STANTON ET AL.: MORPHOLOGICAL DNERSITY IN THE A-GENOME COTTONS 521 at College Station, TX, in 1991 for verification of observations. For the accessions grown at Fayetteville, one or more herbarium specimens were prepared as vouchers for each accession. This consisted of a main stem leaf and a flowering branch taken from a typical plant within a 3.3-m row. A complete voucher specimen was not obtained for 42 accessions due to low plant number (low seed viability) or failure to flower under field conditions. Additional accessions were not included for reasons given later, thus, detailed analyses of diversity involved 272 G. arboreum and 84 G. herbaceum accessions. A list of included accessions identified by their NPGS numbers may be obtained through GRIN or from the corresponding author. Passport Data When available, passport data for each accession was used to assign a geographic origin to the accession. For purposes of analysis the countries of origin were grouped as follows. Middle East: Iraq, Sudan, and Turkey; Central Asia: Afghanistan, Iran, Pakistan, Russia, and Uzbekistan; East Asia: China, Japan, and Korea; Southeast Asia: Burma, India, Indonesia, and Philippines; Southern Africa: Africa, Botswana, South Africa, and Zimbabwe; unknown: no origin listed or the origin listed is outside the range of A-genome cotton cultivation. Morphological Analysis Fifty-three characters comprising 41 morphological traits and 12 proportional characters expressed as percentages or ratios were evaluated. Acronyms and descriptions of the characters are defined in Table 1. Color and boll measurements were taken on live plants; other measurements were made on herbarium specimens or harvested fiber and seed samples. Characters were selected from an assessment of potentially important variation based on initial observations of the range within the collection and on published taxonomic treatments (Hutchinson et al., 1947; Fryxell, 1979). Ratios of selected primary characters were used to describe relative shape of plant parts. Fiber characteristics were measured by STAR LAB, Knoxville, TN, on samples obtained from field-grown plants in 1988, 199, and At this initial level of characterization of the many accessions, it was not practical to test for genotype-environment interactions. Data Analysis Multivariate techniques were used to assess the similarities among accessions and to evaluate morphological parameters contributing to the variation in each species. Quantitative morphological characters that were not invariant or highly correlated to another character were evaluated using principal component analysis (PCA). Data were standardized by character (mean =, standard deviation = 1). Principal components were calculated from a matrix based on correlations between the characters. A cophenetic correlation value was used to evaluate results from the principal component analyses. This value measures the correspondence between a similarity matrix based on the original data and a similarity matrix generated based on the models developed (Wiley, 1981). Specific procedures for PCA were executed with the microcomputer program NTSYS (Version 1.7) (Rohlf, 1992). RESULTS The available passport data for the A-genome accessions give an indication of the geographic regions represented in the collection. Nearly 6% of the G. arboreum accessions originated from East and Southeast Asia while only 1% came from Central Asia. No accessions originated from Africa. The origins of 3% of the G. arboreum accessions are unknown. Thirty-two percent of the G. herbaceum accessions came from Central Asia, 18% from Southeast Asia, 12% from the Middle East, and 7% from East Asia. Only 11% originated in Southern Africa where the only wild populations of this species are extant. The origins of 19% of the accessions are unknown. Because of the lack of basic characterization data for the accessions in the NPGS Asiatic Cotton Germplasm Collection, preliminary observations were made on all accessions for which viable seed could be obtained. Twenty-three accessions in the collection were determined to be G. hirsutum or G. barbadense, and 17 appeared to be segregating populations of interspecific crosses between G. arboreum and G. herbaceum. Fifteen accessions recorded as G. herbaceum were identified as G. arboreum, and 11 accessions recorded as G. arboreum were identified as G. herbaceum. These accessions will be assigned new numbers in the appropriate species collection (GRIN, 1989). Sixty-seven accessions exhibited morphological variants that we fixed by selfpollination of individual plants. These variants within an accession will retain the original accession number plus a subclassification in the Germplasm Resources Information Network (GRIN). Passport data, as far as known, and our characterizations for each accession in the Asiatic collection are available through GRIN (Percival, 1987; GRIN, 1989). This base information should provide for a more orderly use of the collection. For purposes of subsequent analyses, accessions we identified as G. hirsutum or hybrid swarms were excluded. Mislabeled accessions were included after being appropriately labeled, and the stable variants were included as separate accessions. This resulted in a total of 356 entries being analyzed for morphological diversity. Summary statistics for the 356 accessions are shown in Table 2. Since the ranges for every character overlap between species, no single character serves to delineate these species. However, means for 41 of the characters differed significantly when compared by a t-test (P ~.5) (SAS, 1989). The range extremes of morphological characters were greater for G. arboreum in 29 cases, for G. herbaceum in 16 cases, and equivalent in eight cases. Two characters, petal color and petal spot, were invariant in G. herbaceum. In spite of the two invariant characters and the range differences, the mean coefficient of variation for G. herbaceum (35.6%) was not significantly different from G. arboreum (37.6%). Twenty-six quantitative characters were selected for use in the principal component analyses. In most cases, characters that were highly correlated were not both used. Four characters that were used to compute ratios, and, therefore, obviously correlated to the ratio computed, were retained to capture both the size and shape components of variation. A few accessions never flowered and, therefore, have missing flower and fiber data. If patterns of missing data corresponded to any relationships among the accessions (e.g., earliness or photoperiod

6 522 CROP SCIENCE, VOL. 34, MARCH-APRIL 1994 Table 2. Summary statistics for measured and derived cbaracterst for Gossypium arboreum and G. herbaceum. G. arboreum G. herbaceum Structure/Parameter N Range Mean so N Range Mean so Flower Petal color s.o.7 Petal spot PoUen color Petal length (em) Petal/bract(%) Leaf Number of lobes s.s s.8 1. Color Lm length (em) Lm width (em) Ls length (em) Lobe shape(%) Lobe length(%) * 1.1 Sinus depth(%) * * 7.1 Glands * * 6.9 Stipule (em) Bract Sunred * 1.1 Length (em) Width (em) Number teeth * 2. Teeth length (mm) Teeth/bract (%) ss Width/length (%) * * 14.9 Nectary Leaf nectary o.8.3 Flower nectary Nectary area (mm 2 ) o.8.6 Pubescence Leaf * o.8.4 Stem, 2 layers * Stem rating Main stem (mm) Minor stem (mm) o *.6 Simple hairs (mm) SteUate hairs (mm) BoD Length (em) Width (em) Width/length (%) * s.o 8.3 Seed/Fiber Fuzz density Lint color Seed index (g) * 2. Naked seed wt. (g) Lint percent * * 6.4 E1(%) Micronaire s.o 1.1 T1 (kn mlkg) * * 3.3 SO% SL (mm) * * % SL (mm) A (mm 2 /mm 3 ) * * 74.3 Au (mm 2 /mm 3 ) * 96.4 Au-A (mm'/mm 3 ) * Maturity(%) Immaturity ratio * *.4 Perimeter (pm) Wt. fineness (pg) wan thickness (pm) Means between species are significantly different (P =.5). t See Table 1 for definitions of characters. requirements imposed by similar origins), the results Principal component analysis was performed on the would be biased (Jackson, 1991). The cophenetic correla- combined data set and individually on each species in tion between PCA with and without fiber, seed, and boll order to examine infraspecific diversity and to extract data was.94, indicative of a very good fit between important components of variation within each species the two models. We chose to use the fiber data, even (Table 3). For the G. arboreum and combined data sets, though they were absent for approximately 27% of the cophenetic correlations (.86 and.87, respectively) accessions, because human-mediated selection based on indicated a good fit between a distance matrix based on fiber traits probably contributed to differences within the original data and a distance matrix based on 25 each species. principal components, the number of principal compo-

7 STANTON ET AL.: MORPHOLOGICAL DIVERSITY IN THE A-GENOME COTTONS 523 Table 3. Characters used in the principal component analyses of G. arboreum and G. herbaceum, the loading of each character on principal components I, ll, and ill, and the variance explained by each component. Combined data sett G. arboreum G. herbaceum Charactert II m II m II m Bract length Bract shape Bract tooth length BoU length BoU shape BoU width Bt length/bract length E Glands Leaf color Lint percent Lm length Lobe length Lobe shape Micronaire Naked seed weight Nectary area Number of teeth Petal length/bract length Sinus depth SteUate pubescence Stem pubescence, main Stipule Sunred T % span length Variation explained (%) Cumulative variation (%) t G. arboreum and G. herbaceum data combined before standardization. t See Table 1 for descriptions. model accounted for 34% of the variance (Table 3), and the differences between the two A-genome species are apparent in plots of the first two PCs (Fig. 1). An examination of characters with the highest factor loading show the first component to be primarily based on charac- ters that have been used to differentiate the species (no. teeth, tooth ratio, bract length, bract width/length, leaf nents (PCs) necessary to capture 1% of the variation in each data set. In contrast, the cophenetic correlation value for G. herbaceum (.9) indicated a very good fit, and fewer (19) PCs than in the combined or G. arboreum models were required to obtain 1% of the variation in the data set. The first three components in the combined species 1.5 G. herbaceum o G. arboreum -1.~ ~------~------~ r Principal Component 1 Fig. 1. Principle component analysis (PCA) of 314 A-genome cottons; plot of individual accessions on first two PCA axes. PCA data shown in Table 3.

8 524 CROP SCIENCE, VOL. 34, MARCH-APRIL _,------;A~Ce_n_tra_I_As_la---,<: , 6 Eastern Asia Mid East C\ <\? Southeasterp Asia c: Q) 8. E 8 a; a. ~ -.5 Q.... <:- ~ <: Principal Component 1 Fig. 2. Principle component analysis (PCA) of 169 G. arboreum accessions; plot of individual accessions on first two PCA axes. PCA data shown in Table 3. Accessions are designated by region of origin. Seventy-four accessions with unknown origins are not shown. sinus, lobe shape, boll shape) (Fryxell, 1979; Hutchinson and Ghose, 1937). Other characters contributing to differences between the species were rnicronaire and lint percent. For G. arboreum, the first three components explained 3% of the variation (Table 3). Characters that appeared important to the first component were fiber (rnicronaire, lint percent), leaf (sinus, lobe shape), and bract (length, petal length/bract length) traits. Plots of PC 1 and PC 2 revealed little geographical alliance, except accessions from eastern Asia tended to fall below the mean for PC 2 (Fig. 2). Accessions from southeastern Asia were distributed throughout the plot, perhaps reflecting the variation in this group of accessions from the purported geographic center of diversity (Wendel et al., 1989). Too few of the accessions had racial designations for a robust test of racial validity, and natural infraspecific groups were not suggested by the data; rather, accessions of G. arboreum are represented by a fairly even dispersion of points. The first three PCs explained 41 % of the variation among the G. herbaceum accessions (Table 3). In the first component, the factors with the highest loading (coefficients >.5) were all positive, indicating that size was an important factor (Pimental, 1979). For example, boll length and width, but not the ratio of boll width to boll length, had high factor loadings; thus, absolute size but not shape was a major variable. In the second component, most factors with the high loadings were ratios of characters, indicating the importance of shape. Variability within G. herbaceum appears to be more organized within the collection. Of the races identified among the accessions examined, or judged to fit racial descriptions, G. herbaceum subsp. africanum accessions were near the means for PC 1 and PC 2 (Fig. 3a). With one exception, the accessions obtained from southern Africa reflect this same mid-range (Fig. 3b). One of the outlier points was an accession listed as being from a research group in Zimbabwe, but it is also identified in the collection as kuljianum, a western China race. Middle Eastern accessions are clustered above the means for PC 1 and PC 2. Only one of these accessions had a racial designation (persicum). Most of the other persicum accessions were scattered around the mean of PC 2 rather than above it (Fig. 3a). Accessions of race wightianum clustered in the lower right. DISCUSSION At the initiation of this study, the NPGS Asiatic Cotton Germplasm collection contained 254 accessions of G. arboreum and 138 accessions of G. herbaceum. The majority of these were originally obtained from other germplasm banks. For most accessions, only some of the following information was known, originating germplasm bank, collector and collection number, country of origin, year of collection, racial classification, and common name. Errors in documentation were evident in the original passport data of some of the accessions donated to the collection. We have noted these errors and, to the extent possible, made corrections in the collection documentation in GRIN. Morphological analyses demonstrated that G. arboreum and G. herbaceum differ significantly in 41 of the 53 traits measured, although not a single character serves to completely distinguish the two. Several of these characters have previously been used to distinguish between the two species (bract: width/length, tooth length, and #teeth) (Hutchinson et al., 1947; Fryxell, 1979). Other characters, including leaf sinus, number of gossypol glands, and sunred, which varied significantly between species, have been mentioned as differences between the

9 STANTON ET AL.: MORPHOLOGICAL DNERSITY IN THE A-GENOME COTTONS 525 C\1... c: 4) c: 8. ~ 1 A,.r-=.~a_fri_c_a_n_u_m.5 as. ~ -.5 D.. A kuljlanum 6 perslcum wlghtlanum u\ ,----': -A--::;;:------f' j C\ c: 4) c: o Southern Africa A Central Asia 6 Eastern Asia Mid East "(} Southeastern AsiB.t ol}o $ <I> ~ "' " -- 6 as 6. " t:. " " c: -.5 't: "~\ " 6 " D.. ol}o " " B Principal Component 1 Fig. 3. Plot of individual accessions of G. herbaceum on first two principle component axes. Principle component analysis (PCA) data shown in Table 3. (A) Plot of 26 accessions designated by racial classification. Forty-four accessions without racial designations are not shown. (B) Plot of 58 accessions designated by origin. Fourteen accessions with unknown region of origin are not shown. species (Harland, 1932; Hutchinson and Ghose, 1937; Silow, 1944; Hutchinson, 1954). Their separation into two groups by Zaitzev (in Fryxell, 1979) has been supported by other studies showing genetic and cytogenetic differentiation (Gerstel, 1953; Phillips, 1961; Silow, 1944; Stephens, 195; Gennuretal., 1988; Wendeletal., 1989). In this evaluation, significantly different means for over three-quarters of the evaluated characters and the multivariate separation of accessions into two overlapping clusters (Fig. 1) further support the separation of these species. The overlap of species distribution in the PC plot may be due to overlap in character ranges and probable introgression between the species during centuries of sympatric cultivation (Silow, 1944; Wendel et al., 1989). Both high levels of infraspecific variability and the overall resemblance of the two species contributed to the absence of complete morphological distinction; thus, a combination of characters is necessary to distinguish between G. arboreum and G. herbaceum. Though taxonomic keys have not used foliar traits to distinguish these species, two leaf characters (lobe shape, sinus depth) strongly influenced the separation of the two species in this analysis. Hutchinson and Ghose (1937) maintained that, because a Mendelian multiple-allelic series affected leaf laciniation, leaf morphology was not a useful taxonomic tool in A-genome cottons; yet in this study several foliar traits appeared to exhibit consistent, though subtle, differences between species. In general, G. arboreum has longer leaves with narrower lobes and deeper sinuses

10 526 CROP SCIENCE, VOL. 34, MARCH-APRIL 1994 than G. herbaceum. Other statistically significant traits, not mentioned in any keys, that distinguish G. arboreum from G. herbaceum include the ratio of petal length to bract length, number of glands, nectary size, and seed size (naked seed wt. and seed index). Species means indicate that G. herbaceum tends to have less anthocyanin, finer lint, and smaller flowers than G. arboreum. Though both species have nectariless accessions, accessions with leaf nectaries but without involucellar nectaries were found only in G. herbaceum. In contrast to the separation of G. arboreum and G. herbaceum based on morphology, analysis of infraspecific variation patterns led to few recognizable groupings within either species (Fig. 2 and 3). These results contradicted expectations based on descriptions of geographic races in the literature (Hutchinson, 195; Hutchinson and Ghose, 1937; Silow, 1944). Most of the difficulty with the original racial classifications stems from their definitions. Rather than being based on overall patterns of morphological resemblance, racial classifications in both species were primarily rooted in geographical ranges. To some extent, the artificiality of these systems was apparent to the original authors (Hutchinson, 1959; Hutchinson and Ghose, 1937; Silow, 1944) who noted that many geographic regions contained extensive variation. Some accessions within each species could be assigned to a specific race, especially within G. herbaceum (wightianum, africanum, and kuljianum). However, most accessions in G. arboreum exhibited characters that could result in assignment to more than one race. Classification difficulties of the accessions may be compounded by inadequate or misleading passport data for geographic origin and race, lack of samples representing all races, and genetic shifts within the collection associated with storage and seed increases. The study, maintenance, and use of the germplasm has essentially ceased as the A-genome cottons have been replaced by G. hirsutum in most of their former cultivated range. It is of interest to ascertain what proportion of this formerly wide-ranging and morphologically diverse genepool has been captured in the NPGS system. While much variability exists within the collection, some morphological types and geographic races are represented by only a few accessions. While the race designations in G. herbaceum appear to have some merit, the diversity or validity of race acerifolium cannot be addressed because this race is not represented in the collection. Fewer than 1 accessions of G. herbaceum subsp. africanum are in the collection. Isozymic and morphological evidence indicates that subsp. africanum bears the most resemblance to a theoretical wild ancestor of G. herbaceum (Hutchinson, 1959; Wendel et al., 1989). Its position in the morphological middle of the plot lends further support to this notion. As africanum is the most morphologically primitive and isozymically diverse form of G. herbaceum, further collections are warranted. The western Chinese race kuljianum, an unusually earlyfruiting G. herbaceum, is represented by a single accession with two morphological types. No representatives of G. arboreum are listed for Africa, and few are from the Middle East. On the other hand, southeastern Asia is reasonably well represented; however, accessions from this region were distributed throughout the PC plot. This may reflect the variation expected from the purported geographic center of diversity (Wendel et al., 1989). Knight (1954, in Endrizzi et al., 1985) published a list of gene symbols and linkage groups for the A-genome cottons representing both common and rare alleles (Silow, 1941). Characteristics not present in the NPGS collection include lintless (AI), virescent (AI and A2), single lobed leaf (A2), brown lint (AI), and some anthocyanin forms (AI and A2). In addition to qualitative traits, we found also that some morphological types have limited representation. These observations indicate that the collection has an incomplete representation of the diversity that was once available. Efforts should be made to collect remnant indigenous seed from portions of the ranges that once were described as comprising races and to obtain better representation within the collection of G. herbaceum subsp. africanum and races acerifolium and kuljianum, and of G. arboreum from Africa and the Middle East region. Additional accessions from these geographic regions and races could contribute additional diversity to the germplasm collection. In spite of the limitations of the A-genome cotton collection, it is a potential source of genetic diversity for G. hirsutum. Introgression from several species into G. hirsutum have successfully contributed genetic variance for quantitative traits such as stress tolerance, fiber quality, and yield (Meredith, 1991). Hybrids of G. hirsutum x G. arboreum have led to selections with earlier maturity and an increased range of fiber traits (Cooper, 1969; Wange et al., 1989). With modern techniques interspecific introgression of A-genome traits into ADgenome cottons may be facilitated by constructing synthetic AD allotetraploids or by crossing with ADD hexaploids (Stewart, 1992). Morphological characteristics of accessions evaluated in this study will be incorporated into the GRIN database. Additionally, we have evaluated much of the NPGS Asiatic cotton collection for resistance to a number of pests (Stanton et al., 1987, 1988, 199, 1992, 1993). 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